1. Field of the Invention
This invention relates to a zoom lens system for use with imaging device, such as a charge coupled device (CCD) and an image pick-up tube, a photographic camera using a silver film, and a projection type television set.
2. Description of Related Art
There have been known zoom lens systems of the type including a negative power first lens group, a positive power second lens group, a negative power third lens group and a positive power fourth lens group. The first lens group is movable axially for focusing the zoom lens system and, however, remains fixed during zooming the zoom lens system. The fourth lens group is stationary. During zooming the zoom lens system, the second and third lens groups are moved axially relative to each other and relative to the first and fourth. One of such zoom lens systems known from, for example, Japanese Unexamined Patent Publication No. 5-297276 has been designed for the purpose of forming an image on a small size of imaging device such as a CCD.
In recent years, projection type television sets with liquid crystal display means installed therein have received much attention. If the zoom lens described above is used as a projection lens of such a projection type television set, the zoom lens does indeed become bulky with an increase in image size, which is always undesirable in view of strong demands for miniaturization of the projection type television set. Further, if a zoom lens is used as a projection lens, it is generally hard for the zoom lens to sufficiently reduce aggravation of distortion. In particular, if a zoom projection lens is applied to the projection type television set, from the standpoint of preventing aggravation of occurrence of irregular color and a quantitative light loss which are caused depending upon lighting systems, the zoom projection lens is desired to be of a telecentric type which makes a principal ray parallel to its optical axis at a side on which an image is reduced. The prior art zoom lens is not of the telecentric type. Further, the prior zoom lens has only a back focal length too short to admit a color separation optical element in an appropriate axial position.